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Thursday, April 17, 2008

Well, here we are at the Bluestone Mine, looking off the to northwest, toward Yerington - more or less. The view is mostly of Mason Valley, with Yerington possibly visible to the left in the middle distance, and the Desert Mountains forming the horizon in the far distance.

At the Bluestone Mine, we first examine one of the 2nd-generation low-angle normal faults, seen in the prospect face above the orange-vested, bending-over, back-to-us geologist. This 2nd-generation fault overall dips about 30 degrees, although here the apparent dip is closer to 45 degrees. In this photo, it's a sharp fault, and you can almost see dip-slip slickensides just a little above and to the left of aforementioned geologist. This second-generation fault cuts first-generation normal faults, like the Singatse Fault. See earlier post. (Also for more references.)

After scrambling around on a hillside or two looking at some sodic-calcic alteration minerals and some intrusive contacts (future post), we finally get to a viewpoint where, looking south, we can see the Bluestone Mine and the main 2nd-generation fault, which is marked by the color change between the brownish, mineralized, hanging-wall rocks and the whitish, intrusive, footwall rocks - and approximately marked by a yellow line someone inserted into the photo.

Singatse Fault:

After even more scrambling around hillsides, up and down, over and beyond, we came to our lunch stop and the Singatse Fault, one of the major 1st-generation low-angle normal faults, which can be seen above in a trench made just to expose the fault. Here, the Singatse Fault juxtaposes sheared quartz-sericite-pyrite-altered intrusive rocks of the Yeringtonbatholith in the hanging wall against shattered Luhr Hill Granite in the footwall (both are Jurassic in age). I dug around in this exposure, but couldn't find any good sense-of-motion indicators - the pieces just crumble and fall apart.

The Singatse Fault runs up the canyon to the west, where it can be seen in old drill roads to the north, marked by a color contrast between the reddish Tertiary volcanic and sedimentary rocks above (hanging wall) and the whitish Jurassic intrusive rocks below (footwall).

Here, someone had conveniently dug out another example of the Singatse Fault, and had equally conveniently placed a flagged rock hammer on the contact. By the time we reached this spot, which is on a pass exposed to the elements, the wind was blowing hard enough to knock small people over, and everyone had huddled into the small pit where the fault was exposed for cover.

Coming soon: sodic-calcic alteration and some intrusive contacts. Also, we'll see the Tertiary unconformity and possibly one more example of the Singatste Fault.